Aramid fibers are highly resistant to heat decomposition, have inherent flame resistance, and are frequently used in working wear for special environments where flame resistance is required. Fabrics made of these fibers are extremely strong and durable, and have been widely adopted for military applications where personnel have the potential to be exposed to fire and flame, such as aircraft pilots, tank crews and the like. There is a need for dyed fabrics that have flame resistant properties even greater than the undyed fabrics or dyed fabrics. Metalinked aromatic polyamide fibers (aramid fibers) are made from high-molecular-weight polymers that are highly crystalline and have either a high or no glass transition temperature.
These inherent desirable properties of aramid fibers also create difficulties for fiber processing in other areas; specifically, aramids are difficult to dye. Fiber suppliers currently recommend a complicated exhaust dyeing procedure with a high carrier (acetophenone) content; the process is conducted at high temperatures over long periods of time and often results in a product having an unpleasant odor. Such dyeing conditions require substantial amounts of energy both to maintain dyeing temperature and for the treatment of waste dye baths.
Polar organic solvents have also been used to swell the fiber or create voids in the fiber structure to enhance dyeability. These procedures involve Bolvent treatments at elevated temperatures with subsequent dyeing. Another source of dyed aramid fiber is producer--dyed aramid yarn, prepared by solution dyeing in which typically a quantity of dye or pigment is mixed with the molten or dissolved polymer prior to extrusion of the polymer or solution into fine fibers; the dye or pigment becomes part of the fiber structure. Solution-dyed fibers are more costly than the undyed fibers due, in part, to the additional costs of manufacture, and must be used in the color provided by the supplier, leaving the user with only a limited choice of colors. Solution-dyed fibers offer relatively good lightfastness, whereas some undyed aramid fibers, particularly Nomex.RTM. (DuPont) yellow following exposure to UV light.
A process has been described by Cates and others in commonly-assigned U.S. Pat. No. 4,759,770 for continuously or Bemi-continuously dyeing and simultaneously improving the flame-resistant properties of poly(m-phenyleneisophthalamide) fibers that includes the step of introducing the fiber into a fiber swelling agent consisting preponderantly of a polar organic solvent also containing at least one dye together with at least one flame retardant, thereby swelling the fiber and introducing both the dye and the flame retardant into the fiber while in the swollen state. The flame resistance/performance properties of fabrics dyed by this process are significantly improved. Limiting Oxygen Index (LOI) values, as described below, may be as high as 41% for simultaneously dyed and flame retarded T-455 Nomex fabric products produced by the process of this invention. As a means of comparison, undyed T-455 Nomex has an LOI of 27%. However, this process involves some equipment not routinely available on most existing processing lines.
Our earlier U.S. Pat. No. 4,898,596 describes a process for dyeing, flame-retardant treating or both dyeing and flame retardant treating aramid fabrics using N-cyclohexyl-2-pyrrolidone as a dye and/or flame retardant-diffusion promoting agent.
Our previous investigations have identified N-cyclohexyl-2-pyrrolidone, in U.S. Pat. No. 4,898,596 and the octylpyrrolidones, in Ser. No. 07/437,397 filed Nov. 16, 1989, as effective agents for promoting diffusion of dyes and/or flame retardant into aramid fibers. While highly effective for most applications, these materials are costly and presently commercially available from only a single source. We have more recently investigated other amide-type compounds and the relationship between compound structure and efficacy in promoting dyeing and/or flame resistance by durable uptake of phosphorus-containing flame retardants. We have now identified and hereby disclose a series of compounds useful for promoting the dyeing and/or flame retarding of aramid fibers or fabrics. These compounds offer the promise of reduced costs and improved effectiveness of methods of dyeing and finishing aramids.
In the course of this investigation, we have studied the relationship between the water solubility of polar organic solvents and their effectiveness as dye diffusion agents or flame retardant diffusion agents for aramids. It is well known that water-soluble polar organic solvents, such as dimethylformamide, dimethylsulfoxide, dimethylacetamide, methylpyrrolidone or ethylpyrrolidone, are effective dye diffusion promoters for aramids when used in a solvent system containing only a minor proportion of water, or no water at all. However, solvent-system dyeing procedures create possibilities of explosion, pollution, and solvent recovery. It is an object of the present invention to provide processes which use dye diffusion promoters as a minor proportion (about 0.5% to 6%) of the dye bath, as dyeing assistants. Such processes reduce or eliminate the problems mentioned immediately above, and can effect a major reduction in cost and a major improvement in convenience.
In studying candidate amide diffusion promoters as assistants for dyeing or flame retarding aramids, we have discovered that water-soluble polar solvents are ineffective when used in a low concentration, such as 0.5% to 6.0% by weight. This is because little of the water-soluble solvent enters into the aramid fiber to promote swelling and diffusion of dye and/or flame retardant into the fiber; the major portion remains in the dyebath, where it is ineffective. We have discovered that for aramid diffusion agents to be effective, they must have low water solubility under the conditions of dyeing, but not be completely insoluble, since some solubility is necessary for the diffusion agent to reach and penetrate the aramid fibers. Thus, a balance between hydrophilic and lipophilic character is necessary. This property can be measured by water solubility tests, but data are not available in the literature for solubility of amide dye diffusion agents at the temperatures and other conditions used in dyeing.
The hydrophile/lipophile balance can be measured approximately in an homologous series of monoamides by certain secondary properties such as molecular weight, number of carbon atoms in the structure, or percent nitrogen content, since the nitrogen-containing amide groups are responsible for the hydrophilic character of the molecule. However, we have chosen to use a standardized dyeing procedure which measures depth of dyeing and the extent of swelling of the aramid fiber, designated as the "Swelling Value". This procedure, and the criteria for its use, are described in detail below.
It is an object of the present invention to provide a process for dyeing an aramid fiber such as Nomex.RTM.. It is also an object to provide a process for simultaneously dyeing and not detracting from the inherent strength of the aramid fibers. It is also an object to provide a process suitable to conventional equipment such as pressure jets, dye becks or similar machines. It is particularly an object to provide a process for the preparation of dyed, "super FR" Nomexs fabrics of high LOI of 37%-44% as described in the Cates et al patent U.S. No. 4,759,770.